im having a heck of a time getting my converted mccolloch chainsaw to run.

here is a thread devoted to my troubles.

walbro carb problems

DON'T PLUG ANY OF THOSE NIPPLES!!!

The nipple on the casing is to provide the pressure pulse to operate the fuel pump in the carb.

I can't tell with certainty from the pictures you posted, but I believe it gets hooked up as follows:

#1 - connect to main fuel supply
#2 - connect to engine crankcase nipple
#3 - connect as a return line back to the tank (normally the return line from the primer bulb)


If I have them wrong, hopefully someone who has done more conversions than me will help.



<Edit>
I just read your latest post on the SPAD forum... Glad to see you've got it straightened out, but I would recommend running a return line from #3 back to the tank. These carbs will sometimes spit fuel out this nipple. If your saw had a primer bulb, it would be connected here, then the output from the primer bulb goes back to the tank. You CAN eliminate the primer bulb, but it makes the engine easier to start. If the diaphram pump is dry, it won't pump, and the primer bulb draws fuel up to the pump.

Don't forget to leave a vent line from your fuel tank to the outside air, or a vacuum in the tank will prevent fuel draw.

okay when i run the #3 back to the vent line of the tank will i need a third line out of the tank for another vent line? i would think the one from the third nipple would be fine for the vent line also...i think it dumps straight to the venturi.

thanks for your help guys

I was thinking the same thing, but when I asked around, I was told it wouldn't work. I don't think #3 connects directly back into the venturi. There is a one-way valve inside the carb. You could probably get away with using a T-connector, but I prefer to use a third vent line from the tank.

would there be any problem with not hooking nipple 3 the return up to anything and leaving the vent line i already have in the tank open? the carb is a pumper so i dont think it would need a return pressure to operate so i think this should work also right?

Yes, this will work, and is common practice by many pilots.

The downside is that you may spit raw gasoline out this nipple.

Also, there is more chance of the diaphram drying out this way.

On one setup I had...I used a return line to tank. When set up right, when you prine it the excess fuel goes back to fuel tank. Nice to keep air out of system also. Capt,n

thanks so much for helping guys. the problem ended up being inside the carb and not with the way i was hooking the fuel lines up. i did my homework and now it runs great. if anyone needs help with a walbro carb let me know, i taught myself quite a lot about them in the last week...hahaha.

Sometimes that's the best way to learn... What was the problem? Let us learn from your experience.

Luckily, all the reading I've done here on Walbro carbs is compatible with my Zama carb.

the problem was that by hooking the fuel lines wrong and trying to start the motor the diaphram had enough pressure to compress the metering lever. i had to bend it back up to make the engine run. it adjusts the fuel needle inside the metering section of the carb.

if you bend the tab up you get the engine to run richer and down makes the carb run leaner. this is in addition to the regular hi and lo needles.

i also learned how to make any walbro carb run directly from crankcase pressure rather than using the special boots that come to mount the carb in a certain place on the motor....pretty neat trick.

I had to do just as you described with my big Tecumseh. It had a bowl type carb that was gravity fed. I'll take a pic and post it in my gallery soon.